1. Field of the Invention
The present invention relates to an OFDM transmitter which transmits an OFDM signal by a frame having a plurality of transmission symbol periods constituted of effective symbol periods for transmitting information bits and guard intervals, and an OFDM receiver which receives the OFDM signal transmitted by the frame.
2. Description of the Related Art
As a system capable of solving a problem of inter-symbol interference (ISI) in a multi-path environment, an orthogonal frequency division multiplexing (OFDM) system has conventionally been a focus of attention.
FIG. 1 shows a configuration of the conventional OFDM transmitter. As shown in FIG. 1, the conventional OFDM transmitter 1 mainly includes a segment section 101, an encoder section 102, a map section 103, an IFFT (Inverse Fast Fourier Transform) section 104a, and a guard interval addition section 104b. Incidentally, the IFFT section 104a and the guard interval addition section 104b constitute an OFDM signal generation section 104.
The segment section 101 is configured to divide an information bit to be transmitted into segments for radio section transmission, and to output the segments to the encoder section 102.
The encoder section 102 is configured to execute error correction encoding processing for each of the segments divided by the segment section 101, and to output the segments to the map section 103.
The map section 103 is configured to map encoded bits in each segment subjected to the error correction encoding processing by the encoder section 102 to a symbol. For example, when 16 QAM is used as a modulation system, the map section 103 maps four “0, 1” signals to one symbol which corresponds to a point out of 16 points on an IQ plane.
And, the map section 103 is configured to map the symbol in a sub-carrier, and to output the symbol to the IFFT section 104a. 
The IFFT section 104a is configured to execute an IFFT processing by using the symbols mapped in a plurality of sub-carriers which have been output from the map section 103, and to output a transmission signal on a time domain.
The guard interval addition section 104b is configured to copy a part of the transmission signal output from the IFFT section 104a, and to add the copy to the transmission signal. Here, a part of the copied transmission signal is a “guard interval”.
The transmission signal is transmitted from an antenna of the OFDM transmitter 100 to an OFDM receiver by a frame in which the guard interval is added to an effective symbol period for transmitting the information bit (symbol).
As a result, for example, even in a multi-path environment shown in FIG. 2, i.e., in an environment in which the OFDM receiver receives indirect waves 1 and 2 in addition to a direct wave, by properly setting an FFT (Fast Fourier Transformation) window position, the OFDM receiver can be configured so that a signal component corresponding to the signals in effective symbol periods, or a signal component of the cyclically shifted version of the signals can be contained in the FFT window.
Thus, when the aforementioned OFDM transmitter 100 is used, since each sub-carrier can be considered in a flat fading environment, it is possible to easily carry out channel compensation processing without any inter-symbol interference.
FIG. 3 shows a configuration of the conventional OFDM receiver. As shown in FIG. 3, the conventional OFDM receiver 200 mainly includes a guard interval removal section 201, an FFT section 202, and a data detection section 203.
The guard interval removal section 201 is configured to remove a guard interval of a frame of a received signal.
The FFT section 202 is configured to execute FFT processing by using the received signal whose guard interval has been removed by the guard interval removal section 201 on a set FFT window, and to output a signal on a frequency domain corresponding to each sub-carrier.
The data detection section 203 is configured to execute channel compensation processing, de-mapping processing, and error correction decoding processing for the signal on the frequency domain subjected to the FFT by the FFT section 202, and to detect an information bit.
In the conventional OFDM receiver 200, when all multi-path components are received by delays equal to or less than a guard interval length, the aforementioned effects can be obtained.
However, when a part of the multi-path components is received after a delay which exceeds the guard interval, there is a problem of inter-carrier interference caused by disturbed orthogonality between sub-carriers in addition to inter-symbol interference.
Accordingly, in order to obtain the aforementioned effects, the guard interval must be set sufficiently long in the conventional OFDM receiver 200. However, the long guard interval causes a problem of a reduction in transmission efficiency.
To solve the problem, there has been developed a system for controlling the guard interval length in an adaptive manner.
FIG. 4 shows a configuration of an OFDM receiver which employs such a system. As shown in FIG. 4, the OFDM receiver 200 mainly includes a guard interval removal section 201, an FFT section 202, a data detection section 203, a channel estimation section 204, an impulse response length estimation section 205, a guard interval length updating amount deciding section 206, and a guard interval length memory section 208.
The channel estimation section 204 is configured to estimate a radio channel from the OFDM transmitter to the OFDM receiver, and to calculate a channel estimation value on a frequency domain (FIG. 6A).
The impulse response length estimation section 205 is configured to estimate an impulse response length in accordance with the channel estimation value on the frequency domain calculated by the channel estimation section 204.
As shown in FIG. 5, the impulse response length estimation section 205 includes an IFFT section 205a and an effective path measurement section 205b. 
The IFFT section 205a is configured to execute IFFT processing for the channel estimation value on the frequency domain, and to calculate a channel estimation value on a time domain (FIG. 6B).
The effective path measurement section 205b is configured to set a multi-path component having predetermined power, i.e., a multi-path component having power which exceeds reference power, as an effective path, among the channel estimation value on the time domain, and to output the length from first effective path to the last one as an estimated impulse response length.
The guard interval length updating amount deciding section 206 is configured to compare the estimated impulse response length calculated by the impulse response length estimation section 205 with a current guard interval length, and to decide an updating amount of the guard interval length.
The guard interval length updating amount deciding section 206 is configured to notify the decided guard interval length updating amount to the OFDM transmitter through a feedback signal, and to store the updating amount of the guard interval length in the guard interval length memory section 208.
The guard interval removal section 201 is configured to remove the guard interval in accordance with contents stored in the guard interval length memory section 208.
Incidentally, functions of the FFT section 202 and the data detection section 203 are similar to those of the FFT section 202 and the data detection section 203 shown in FIG. 3.
FIG. 7 shows a configuration of an OFDM transmitter which employs the aforementioned system. As shown in FIG. 7, the OFDM transmitter 100 mainly includes a segment section 101, an encoder section 102, a map section 103, an OFDM signal generation section 104, a guard interval point number memory section 105, a guard interval point number deciding section 106, and an in-frame bit number memory section 108.
As shown in FIG. 8, the OFDM signal generation section. 104 includes an IFFT section 104a and a guard interval addition section 104b. 
The IFFT section 104a is configured to execute IFFT processing by using symbols mapped in a plurality of sub-carriers output from the map section 103 on a set FFT window, and to output a transmission signal on a time domain.
The guard interval addition section 104b is configured to add a guard interval having the number of guard interval points output from the guard interval point number deciding section 106 to the transmission signal on the time domain output from the IFFT section 104a. 
The guard interval point number memory section 105 is configured to store the current number of guard interval points.
The guard interval point number deciding section 106 is configured to decide the number of guard interval points to be set in a transmission symbol period thereafter, in accordance with a feedback signal (updating amount of a guard interval length) from the OFDM receiver 200 and the current number of guard interval points from the guard interval point number memory section 105.
The guard interval point number deciding section 106 is configured to transmit the decided number of guard interval points to the guard interval addition section 104b and the guard interval point number memory section 105.
The in-frame bit number memory section 108 is configured to store a total in-frame bit number of the transmission signal.
The segment 101 is configured to divide an input information bit into predetermined segments in accordance with the total in-frame bit number.
The encoder section 102 is configured to execute predetermined error correction encoding processing for each segment in accordance with the total in-frame bit number.
However, in the conventional OFDM transmitter 100 and the conventional OFDM receiver 200, when the guard interval length is changed in each symbol period, the frame length of the transmission signal is changed. Consequently, when multi-user access is considered in a time division system, there is a problem in that band allocation to users becomes very difficult.